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There's a lot of coal in the ground. In 2006, the U.S. Energy Information Agency estimated that global coal reserves (i.e., the total quantity of coal that can be economically produced) stood at roughly 840 gigatonnes. That's enough to supply the world for more than 130 years.

If an additional 4 trillion tonnes were extracted without the use of carbon capture or other mitigation technologies atmospheric carbon-dioxide levels could quadruple--resulting in a global mean temperature increase of between 5 and 10 degrees Celsius."

Reserves, however, are a notoriously misleading metric. Strictly speaking, reserves are the quantity of a commodity that can be economically produced at current prices and with current technology. Over time, reserve estimates tend to increase because both the price of the commodity and the technological ability to recover that commodity change. If the market price increases, for example, while the recovery cost decreases, the reserve base will increase. So, reserves are a lower-bound estimate of the total resource that exists. In the case of coal, if we were to include coal seams that are currently too deep for conventional mining then global coal reserve estimates would probably increase by an order of magnitude.

But this raises two vital questions: (1) Will it ever be economical to extract such additional coal; and (2) if it were, is it a good idea to do so?

Let's take these questions in order.

Deep coal seams are labeled unminable because the cost of extraction outweighs the value of the coal itself. But there's a way to harness the potential energy stored in the coal without mining for it. Specifically, coal can be gasified underground, and the products of gasification can be extracted through wells in a manner similar to the extraction of natural gas. Coal gasification involves a process known as partial oxidation, a form of combustion that occurs when there is insufficient oxygen. During gasification, coal, oxygen, and water are combined in the appropriate ratios such that the carbon in the coal is converted into carbon monoxide and the hydrogen in the water is converted to hydrogen gas. A key element of this process is that the carbon is not converted all the way to carbon dioxide. The product of this reaction, a mixed gas of carbon monoxide and hydrogen, is known as syngas, which is widely used as feedstock for producing chemicals (such as fertilizer) and also can be burned to generate electric power.

How is coal gasified underground exactly? First, an appropriate quantity of oxygen (or air) and steam is injected into a coal seam through vertically or horizontally drilled wells. This combination of gases will, in turn, gasify the coal. The syngas can then be extracted through separate production wells that are drilled "downstream" of the original injection sites.

In effect, such a process amounts to starting a controlled fire underground, which sounds dangerous. But, if done properly, it's not. Controlled underground coal gasification projects are intentionally set below the water table, meaning they can be extinguished by simply turning off the injection of air and/or steam. When the injection stops, the surrounding groundwater flows back into the coal seam and quenches the burn.

Now, the second question: Is underground coal gasification a good idea?

It's been estimated that around 4 trillion tonnes of otherwise unusable coal might be suitable for underground gasification. If true, then the economic development of this process would expand coal reserves by a factor of about five. Such an expansion would be both good and bad. From the perspective of maintaining a prodigious and affordable energy supply, gasification would be a boon. But from a climate change perspective it could be a nightmare. If just current conventional coal reserves were fully combusted, the concentration of atmospheric carbon dioxide would approximately double. But if an additional 4 trillion tonnes were extracted without the use of carbon capture or other mitigation technologies, atmospheric carbon-dioxide levels could quadruple--resulting in a global mean temperature increase of between 5 and 10 degrees Celsius.

Here's good news though: Carbon dioxide produced from gasification operations can be mitigated. Specifically, after syngas is produced, it can be reacted with steam to convert the carbon monoxide, hydrogen, and water into carbon dioxide and additional hydrogen. The hydrogen can then be separated and used in any number of ways, while the carbon dioxide can be injected into a different geological stratum for permanent storage. This process, called carbon capture and storage, is actively being studied and developed for multiple applications.

Underground coal gasification coupled with carbon capture and storage holds the promise of producing an effectively limitless supply of low-carbon, fossil-fuel derived energy. While the economics of underground gasification are not yet obvious, several studies--including an excellent pair by Julio Friedman of Lawrence Livermore National Laboratory (available here and here)--suggest that under the right circumstances the economics could be very attractive. There are several reasons why the economics look good. For instance, doing the gasification underground obviates both the very expensive surface gasification facilities and the coal handling equipment. Indeed, according to Friedman, syngas produced through underground gasification is 25-50 percent less expensive than surface-produced syngas. Furthermore, it's easier and more efficient to transport gas than it is to transport solid coal.

Underground coal gasification has been pursued for nearly a century and today there are nearly 20 such projects in various stages of planning and development. Eskom, a South African utility company, is planning to build a 2.1 gigawatt combined cycle plant to be run entirely on syngas from underground gasification. China and India are also aggressively pursuing research and development of underground gasification.

Underground gasification paired with carbon capture and storage technology has the potential to provide significant quantities of clean coal-derived energy. But without carbon capture, the climate would be put at severe risk. It's therefore vital that development of underground gasification be done in tandem with development of carbon capture technologies. The alternative could be disastrous.